What is your required budget, resolution and repeatability? Environment can be a factor too. As if a small rack and pinion(Shooting Star DRO's use this), could be set up in sections to any length. Continuous timing belt secured at each end will idlers at the encoder cog pulley. Trav-a-dial's concept used a wheel riding directly on a face of ground surfaces that was indicator or encoder type.
You might consider a Red Lion PAXI or similar counter display. They have capability to read quadrature encoder signals, scale them and output relays controlled via set points.